Materials with excellent stretchability and elasticity are needed to manufacture a variety of disposal and durable articles such as, for example, incontinence pads, disposable diapers, training pants, sports apparel, general apparel and furniture upholstery.
Disposable articles are typically elastic composite materials prepared from a combination of polymer film, fibers, sheets and absorbent materials as well as a combination of fabrication technologies. Whereas the fibers are prepared by well known extrusion/spinning processes such as spun bonding, melt blowing, melt spinning and continuous filament winding techniques, the film and sheet forming processes typically involve known bulk extrusion and coextrusion techniques, e.g., blown film, cast film, profile extrusion, injection molding, extrusion coating, and extrusion sheeting.
Block polymers, especially styrenic block copolymers (SBCs), generally are elastomeric materials that exhibit excellent solid-state elastic performance attributes. But the most common unsaturated block copolymers, styrene-butadiene-styrene triblock polymers (SBS), tend to exhibit mediocre thermal stability, especially in the molten state. In addition, SBS polymers readily form gels due to cross-linking at temperatures necessary to pass these materials through the fine holes of textile or nonwoven dies at commercial rates or draw-downs. Furthermore, drawing of SBS polymers as fibers at temperatures below their cross-linking temperature cannot be done at commercially viable levels due to ductile or melt fracture of the fiber.
Similarly, known partially hydrogenated (or partially saturated) styrene block copolymers (for example, KRATON G block copolymers formerly supplied by Shell Chemical Company, now sold by Kraton Corp.) and Septon (sold by Kuraray) are difficult to melt process and draw into fibers from their pure state. In fact, preparation of fine denier fiber (that is, less than or equal to 40 denier or 78 micron (1 micron=10−6 m) diameter) or thin film (that is, less than or equal to 2 mils) from substantially-neat, partially hydrogenated or partially saturated block polymers is generally not possible at commercial fabrication rates. To overcome characteristic melt processing and drawing difficulties, partially hydrogenated block copolymers are commonly formulated with various additives such as oils, waxes and tackifiers. But in order to achieve good melt processability and drawability, very high levels of low molecular weight additives are typically required. Such high levels of low molecular weight additives tend to compromise strength and elastic properties. In addition, the hydrogenation process required to produce these polymers adds significant further cost.
There are as yet no commercial, low cost materials being produced, especially by the textile spinning, spunbond or meltblown techniques. The present inventors have recognized that it would be advantageous to have an inexpensive elastomeric material with good properties and a process that was economical and could be run at commercial rates.